In the manufacture of polymers or polymer alloys hot extruded strands of polymer are often cooled by passing the strands of polymer through a cooling bath of a liquid, usually water. Typically the cooling bath would be in the order of tens of feet long. The cool wet strands are then drawn from the cooling tank or bath and dried. The polymer strands must be dried before being chopped to pellets. While this sounds relatively easy it is in fact one of the rate limiting steps in the production of polymers or alloys.
In a conventional polystyrene plant the polymer strands are dried by passing the strands over a blower. In theory the blower blows off the water adhering to the surface of the polymer strand. However, at the rates required to operate a world scale plant the drying is often just barely adequate. If there is any difficulty with the dryer then the lines are insufficient. In many instances backup motors are kept available should there be a problem. Generally there seem to be two schools of thought on this arrangement. One school of thought favours a centralized blower with a ducting system through the plant to each of the dryers. This type of system permits the use of high horsepower blowers and some efficiency of scale but is burdened with the cost of a high horse power blower to be held in back up. The other school of thought favours individual blowers for each drying station. Lower powered blowers are required in such a case and there is a lower potential for a total failure of the system. However, such systems may be more inefficient in terms of overall energy consumption per unit of polymer dried.
Generally the type of dryers used in association with drying thermoplastic strands which have been cooled in a water bath are shown schematically in the drawing of U.S. Pat. No. 3,755,526 issued Aug. 28, 1973, assigned to Mobil Oil Company. The patent is of interest in that it teaches chopping the strands to pellets prior to drying.
There have been several attempts to improve the efficiency of pellet dryers. However, these improved dryers tend to be fairly complicated mechanically.
U.S. Pat. No. 4,632,752 issued Dec. 30, 1986 in the name of Friedrich Hunke, discloses a device for cooling and drying polymer strands in which the cooling trough is downwardly sloping from the extruder to the pelletizer. The strands of polymer flow down the trough together with a stream of water. The water is withdrawn from the trough through a drain then air is drawn over the wet strands to remove any water adhering to the strands. The resulting dried strands are then pelletized. The art does not suggest the dryer construction of the present invention. However, the reference is of interest in that it proposes the use of a sucker rather than a blower.
There are several patents in the name of Werner & Pfleiderer which relate to drying pelletized polymer. These are represented by European patent 260 606 dated Mar. 23, 1988 and U.S. Pat. No. 4,570,359 issued Feb. 18, 1986. In these patents the polymer strands are pelletized under water then the pellets and water are drawn up a vertical pipe where the water is separated from the polymer. The references do not suggest the type of apparatus claimed in the present application.
The present patent application seeks to provide a simple suction dryer which may be used to dry polymer strands prior to pelletizing.